fixed x axis not using the correct timestamp

app/src/main/java/ovh/smayzy/i2d_project_15_app/MainActivity.kt

@@ -21,7 +21,6 @@ import com.github.mikephil.charting.data.LineDataSet
 import java.io.BufferedReader
 import java.io.IOException
 import java.io.InputStreamReader
-import java.text.SimpleDateFormat
 import java.util.*
 import android.graphics.Color
 import androidx.core.content.ContextCompat
@@ -51,6 +50,8 @@ class MainActivity : AppCompatActivity() {
     private lateinit var tempCircle: TextView
     private lateinit var humCircle: TextView
 
+    private var startTime: Long = 0L
+
     private val isTesting = true
 
     override fun onCreate(savedInstanceState: Bundle?) {
@@ -139,6 +140,7 @@ class MainActivity : AppCompatActivity() {
             bluetoothSocket?.inputStream ?: return
         }
         val reader = BufferedReader(InputStreamReader(inputStream))
+        startTime = System.currentTimeMillis()
 
         while (true) {
             try {
@@ -152,7 +154,11 @@ class MainActivity : AppCompatActivity() {
                     val hum = parts[1].toFloatOrNull()
 
                     if (temp != null && hum != null) {
-                        val reading = SensorReading(System.currentTimeMillis(), temp, hum)
+                        val reading = SensorReading(
+                            secSinceStart = (System.currentTimeMillis() - startTime) / 1000,
+                            temperature = temp,
+                            humidity = hum
+                        )
                         sensorHistory.add(reading)
                         Log.d(tag, "Store: $reading")
 
@@ -186,9 +192,7 @@ class MainActivity : AppCompatActivity() {
                 position = XAxis.XAxisPosition.BOTTOM
                 valueFormatter = object : ValueFormatter() {
                     override fun getFormattedValue(value: Float): String? {
-                        val time = Date(value.toLong())
+                        return "${value.toInt()}s"
-                        val format = SimpleDateFormat("HH:mm:ss", Locale.getDefault())
-                        return format.format(time)
                     }
                 }
                 granularity = 1f
@@ -235,8 +239,11 @@ class MainActivity : AppCompatActivity() {
         }
 
         tempChart.data = LineData(tempSet)
-        humChart.data = LineData(humSet)
+        tempChart.notifyDataSetChanged()
         tempChart.invalidate()
+
+        humChart.data = LineData(humSet)
+        humChart.notifyDataSetChanged()
         humChart.invalidate()
     }